Earthquakes and Tsunamis

Correct answer: A. Sudden displacement of the sea floor.

Explanation: Tsunami generation depends primarily on the rapid displacement of a large volume of seawater, not simply on how deep a nearby trench is. A powerful undersea earthquake, submarine landslide, or volcanic event can suddenly lift or drop the seabed and transfer energy into the water column. The resulting waves then spread outward and may grow dangerous in shallow coastal water. Thus option A identifies the decisive event. Trench depth may influence regional geometry and wave behaviour, but it is not sufficient by itself to establish tsunami risk. Sand colour and the quietness of an abyssal plain are irrelevant, while ridge length alone does not measure sudden vertical displacement.

Correct answer: A. Subduction, megathrust earthquakes and water-column displacement

Explanation: The governing concept is a tectonic hazard chain. At many ocean trenches, one tectonic plate is forced beneath another at a subduction zone. If the locked boundary suddenly ruptures, a great megathrust earthquake can produce vertical displacement of the seafloor. That movement transfers energy to the overlying water column and may generate a tsunami, which can travel across the ocean and inundate distant coasts. Thus option A gives the complete scientific connection from landform to process and hazard. Coastal sand and wind describe ordinary surface processes, an abyssal plain with quiet deposition does not explain sudden seismic displacement, and colour or temperature alone cannot establish a tsunami mechanism. The risk is conditional, but the process chain is correct.